Excimer gas lasers are part of a family of electronic transition lasers that produce powerful laser pulses having wavelengths in the UV/visible region of the electromagnetic spectrum. The term excimer is used to describe the lasing species which is an excited molecule that is not stable in the ground electronic state. Excimer molecules do not exist in the initial gas mixture but are produced by action of an electrical excitation such as a discharge acting on the initial mixture constituents. This feature distinguishes excimer lasers from other gas lasers.
Of the excimer lasers, those of the rare gas-halide class are the most important because they can be produced with high efficiency using readily available technology such as by the electric discharge method. Examples of rare gas-halide excimer lasers are ArF, KrF, XeC1 and XeF generating UV wavelengths of 193 nm, 248 nm, 308 nm and 351 nm, respectively. Lasers of this type are produced commercially and are used for a wide variety of industrial, medical and scientific applications. Their properties are reviewed in comprehensive summary articles by J. Hecht appearing in Lasers and Applications, Volume II, page 43, December 1983 and by H. Pummer appearing in Photonics Spectra, Vol. 19, p. 73, May 1985.
There are many applications in areas such as spectroscopy, optical diagnostics, materials processing and medicine for which UV or visible laser radiation at more than one wavelength is needed. However the discharge excited rare gas-halide lasers currently known are capable of producing only one wavelength at a time. For example, in order to produce 193 nm radiation typical of the ArF laser, a gas mixture containing Ar and a fluorine donor such as F.sub.2 is used in the laser chamber. To generate another wavelength such as 351 nm radiation from XeF the gas mixture must be changed to one containing Xe and F.sub.2. Therefore a single laser system can produce only one wavelength at a time. For applications requiring two or more UV wavelengths simultaneously, a separate laser system is required for each wavelength, introducing considerable additional cost and complexity.
Moreover, because rare gas-halide lasers generate very short duration pulses on the order of 10 nsec, synchronization of the laser pulses from different laser systems producing different UV wavelengths is a serious problem. To supplement the UV laser, other applications require a separate visible laser for spatial alignment or where safety considerations require that the laser beam be seen by the eye.
J. M. Hoffman, et al have disclosed in Applied Physics Letters, Vol. 28, p. 538, May 1, 1976, the feasibility of achieving simultaneous laser oscillation on the 193 nm ArF transition and the 248 nm KrF transition. However, in that work laser excitation was provided by a high energy electron beam accelerator in which a beam of electrons having an energy of 2 MeV was confined by an externally applied axial magnetic field so as to guide the electron beam longitudinally through the laser chamber. This laboratory means of providing excitation was capable of being fired only ten times per day, and is so complex as to preclude use in practical applications. Means to generate more than one UV wavelength at the same time from a single repetitively pulsed, discharge excited rare gas-halide laser, or to generate a UV wavelength and a visible wavelength simultaneously from such a laser are unknown in the prior art.